Q-factor switching method and apparatus for detecting nuclear quadrupole and nuclear magnetic resonance signals

ABSTRACT

A probe ( 80 ) for irradiating a sample with RF energy during transmitting periods and detecting an NQR or NMR signal from a substance contained within the sample during receiving periods. The probe ( 80 ) comprises a variable impedance unit ( 20 ) for changing the Q-factor of the probe and a probe coil. The probe ( 80 ) is responsive to powerful RF pulses applied thereto to excite an RF magnetic field in the probe coil during the transmitting periods. The variable impedance unit ( 20 ) is controllable to provide a Q-factor for the probe ( 80 ) at: (i) an optimal level during a prescribed transmitting period of an RF pulse for irradiating the sample with said RF energy; (ii) a minimal level during a prescribed recovery period immediately following said transmitting period to rapidly dampen transient signals from the probe; and (iii) a maximal level during a prescribed receiving period for detecting an NQR or NMR signal from the target substance if present, immediately following the recovery period.  
     A method for detecting an NQR or NMR signal within a sample using the probe ( 80 ) is also described.

FIELD OF THE INVENTION

This invention relates to nuclear quadrupole resonance (NQR) and nuclearmagnetic resonance (NMR) detection equipment, and more particularly toan apparatus and method for changing the Q-factor (quality factor) of aprobe used in NQR and NMR detection equipment.

Within this specification the term “substance” is taken to mean thosematerials which respond to the NQR and NMR phenomenon. For a discussionof the NQR phenomenon, regard should be made to our co-pendingInternational Patent Application PCT/AU00/01214, which is incorporatedherein by reference.

Throughout the specification, unless the context requires otherwise, theword “comprise” or variations such as “comprises” or “comprising”, willbe understood to imply the inclusion of a stated integer or group ofintegers but not the exclusion of any other integer or group ofintegers.

BACKGROUND ART

The following discussion of the background art is intended to facilitatean understanding of the present invention only. It should be appreciatedthat the discussion is not an acknowledgement or admission that any ofthe material referred to was part of the common general knowledge as atthe priority date of the application.

Nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR)are methods widely used for the detection and investigation of variouschemical compounds. These methods are also successfully used fordetecting the presence of specific substances, such as explosives andnarcotics.

The probe of a pulsed NQR (or NMR) detection system is a deviceproviding interaction between the radio frequency (RF) field of aresonant RF transmitter and a particular substance that is targetedwithin a sample for detection of NQR (or NMR) signals generated as aresult of the NQR (or NMR) phenomena, as well as interaction between theRF field response from the target substance and the receiving part ofthe NQR (or NMR) detector. Strong RF pulses, typically with hundreds ofwatts of power are used. In practical NQR devices, when detectingspecific substances (for example explosives and narcotics), the power ofRF pulses can reach several kW.

FIG. 1 illustrates a conventional system for detecting NQR (or NMR)signals from a target substance. For NMR a magnet is required but thisis not shown in FIG. 1. A transmitter unit 60′ and a receiver unit 50′are connected to a probe 80′ through a duplexer and matching circuit 40′which switches the probe 80′ between a transmit mode and a receive mode.The transmitter unit 60′ generates RF pulses and applies the pulses tothe probe 80′ during a transmitting period when in the transmit mode toirradiate a sample with RF energy and excite nuclei of any targetsubstance contained within the sample. The pulses have a frequencycorresponding to the resonant frequency of the nuclei of the substanceto be detected and the probe 80′ is tuned to this resonant frequencytypically by a tank circuit to optimise the Q-factor of the probe foroptimal detection. After the RF pulse is applied, the probe 80′ candetect the NQR (or NMR) signal. This signal is received by the receiverunit 50′ during a receiving period when in the receive mode and isprocessed by a control and signal-processing unit 70′, which alsogenerates all control and RF signals.

Strong radio frequency (RF) pulses applied to the probe producetransient signals (“ringing”). This results from the accumulation ofenergy in the circuit of a probe after the impact of RF pulses. Thisremaining RF energy must be dissipated before a probe can be effectivelyused to receive the NQR (or NMR) signal. After the probe has rung down,the NQR (or NMR) signal from the sample can be detected.

The duration of these transient signals, which determines the length ofthe recovery period of a probe, can be quite considerable—from severalhundred microseconds to several milliseconds. This is particularlyapparent when detecting low frequency NQR samples within a high Q-factorprobe coil.

NQR frequencies of many significant explosive and narcotic substancesare found in the low frequency range (0.1-6 MHz) and need to be detectedwithin timeframes of 300 μs to 1.2 ms after irradiation of a samplecontaining same with RF energy for determining their existence. Henceringing can present a major problem for detecting NQR signals lying inthis low frequency range. Low frequency NMR and Magnetic ResonanceImaging (MRI) are also important for biological and medical research, aswell as for some other purposes, and thus ringing also presents aproblem with the detection of low frequency signals in thesetechnologies as well.

In order to overcome this problem the signal-to-noise (SNR) ratio in theprobe needs to be increased. This can be achieved by using high qualityprobe coils having a Q-factor ranging from between several hundred toseveral thousand.

The time constant of a tank circuit for a probe is generally expressedby: ${\tau = \frac{Q}{\pi\quad f}},$where Q is the quality factor and f is the resonant frequency. Thus inthe case of high Q (for example around 1000) of the tank circuit, therecovery period of the probe after the irradiation of the sample withthe powerful RF pulse is very long.

In the art, the requirement for a long recovery period of the probe fordissipating the transient signals prior to being able to detect NQR (orNMR) signals during the receiving period results in causing aconsiderable decrease in the detection sensitivity. Firstly, the induceddelay in switching-on the receiver system to provide for the recoveryperiod of the probe results in a part of the useful signal energy in anyresponsive NQR (or NMR) signal being lost. Secondly, this delay imposesserious time limitations when using multi-pulse sequences. For example,when using the steady-state free-precession (SSFP) or spin-lockingspin-echo (SLSE) type sequences, the best detection sensitivity isachieved when the pulse spacing is optimised, which is determined by therelaxation parameters for each substance. When the recovery period ofthe probe is long, the optimum pulse spacing cannot be achieved in mostinstances, and this leads to subsequent losses in the detectionsensitivity. Consequently it is desirable to get the resonant probe toring-down as soon as possible during the recovery period.

A very high Q for a tank circuit is also undesirable during thetransmitting period. With a high Q, the time constant of the tankcircuit can be too long and the leading edge of the pulse envelope doesnot have time to develop. This results in the amplitude of the RF pulsenot necessarily being able to reach its maximum value in the requiredtime. The shape of the pulse then gets distorted and becomes“triangular” which is not always desirable.

The increase in the pulse duration leads to a reduction in its spread inthe frequency domain, and consequently the RF pulse bandwidth can thenbecome narrower than the NQR (or NMR) resonance line. In this case theresonance line will not be fully excited, which will make the SNR lower.

Too higher Q also limits the effectiveness of amplitude, frequency orphase modulating the pulse. Therefore, for the efficient NQR (or NMR)signal detection in many practical applications, such as detecting thepresence of specific substances, the value of the Q-factor during thetransmitting period must be lower than during the receiving period.

Various techniques have been used to reduce the ring-down time of theprobe and hence the recovery period. One of the more widely used is theresistive damping technique based on the use of a resistive dampingelement, which is electrically coupled to the probe with diodes. In thetransmit mode this resistive element damps a probe during thetransmitting of the RF pulse and for some time after it, keeping a lowQ-factor. When the amplitude of the transient signals reaches the lowerthreshold voltage, a high maximum Q-factor is then provided. Adisadvantage of this method is the necessity to use a very low Q-factorto achieve a rapid diminution of the transient signals. Due to lossescaused by this element, additional increased pulse power is needed. Inaddition, due to Johnson noise, the resistive damping element in theprobe can reduce the SNR.

Techniques based on Q-switched damping are also known. This methodinvolves active damping by switching the total Q-factor from a highmaximum Q-factor during the transmit mode to a low Q-factor during thering-down period, and back to a high maximum Q-factor during the receivemode. Q-switched damping uses actively switched elements (such astransistors, actively switched diodes, triacs or thyristors). When thetransistors or diodes are used, a parasitic charge may be injected intothe probe via the parasitic capacitance of these elements and so causethe probe to ring anew unless appropriate precautions are taken. Otherswitching elements (such as a triac or thyristor) switch themselves offafter a certain recovery time. They do not require a switch off controlsignal. Therefore no charge is injected and no new ringing appears whenthe damping is removed. However, these devices offer less control.

A similar effect can be achieved when using the so-called “slow”transistor. This “slow” transistor exhibits a response time betweenreceiving a switch-off signal and actually switching off. This responsetime is of the order of the damped ring-down time or recovery period ofthe probe. However the use of such a transistor requires cascadeconnection of other elements, such as resistors or/and diodes, whichdiminish the efficiency of damping and are a source of additional noise.Unfortunately, the use of actively switched elements for Q-switcheddamping of the resonance circuit is limited by the maximum voltagecapacity of these elements. In practice it is very difficult to find asuitable device capable of switching more than 1000V. For many practicalpurposes RF voltage can exceed this value considerably, which inevitablyleads to the breakdown of the actively switched element. It is possibleto find some kinds of actively switched elements, which have a maximumvoltage capacity higher than 1000V, however usually this voltage stillis not sufficient for practical use. There are also other reasons as towhy these elements do not provide fast and efficient damping of theprobe, making them unsuitable for use in NQR and NMR applications.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide for changing the Q-factorof a probe in nuclear quadrupole resonance (NQR) or nuclear magneticresonance (NMR) detection for the purpose of optimally sensing NQR orNMR signals from a target substance irradiated with RF energy, withoutsome or all of the disadvantages associated with previous detectionmethods and systems.

It is a preferred object of the invention to accomplish the detection ofNQR or NMR signals from the target substance whilst using a high Q coilin the probe and a high RF voltage during the time that high power RFpulses are applied to the target substance.

It is a further preferred object of this invention to provide optimalparameters for transmitting the RF pulses during the transmit mode andhigh sensitivity of the system during the receive mode.

In the present invention, these objects are achieved by changing theQ-factor of the probe to different Q-factors during: (1) thetransmitting period of the pulse, (2) the recovery period immediatelyafter the transmitting period and (3) the receiving period immediatelyafter the recovery period in methods or apparatuses for NQR or NMRdetection.

Thus, in accordance with one aspect of the present invention, there isprovided an apparatus for changing the Q-factor of a probe for an NQR orNMR apparatus including Q-factor setting means for setting the Q-factorof the probe and Q-factor changing means for changing the Q-factor ofthe probe for detecting an NQR or NMR signal from a target substancewithin a sample irradiated with RF energy wherein the Q-factor changingmeans is controllable to change the Q-factor of the probe to:

-   -   (i) an optimal level during a prescribed transmitting period of        an RF pulse for irradiating the sample with said RF energy;    -   (ii) a minimal level during a prescribed recovery period        immediately following said transmitting period to rapidly dampen        transient signals from the probe; and

a maximal level during a prescribed receiving period for detecting anNQR or NMR signal from the target substance if present, immediatelyfollowing the recovery period.

Preferably, the probe has an impedance that can be varied to achieve aQ-factor of minimal orders of magnitude during the recovery period and aQ-factor of high orders of magnitude during the receiving period.

Preferably, the Q-factor changing means has low reactance for activelychanging the Q-factor of the probe without injecting a parasitic chargethereon.

Preferably, the optimal level is sufficiently low to reduce theprescribed recovery period to a period in which the transient signalsmay be dampened and to develop the leading edge of the pulse envelope ofsaid RF pulse during said transmitting period; the optimal level is alsosufficiently high to reduce the bandwidth of said RF pulse during saidtransmitting period and so mitigate the power expended in amplifying thepulse envelope over the bandwidth; and the maximal level is sufficientlyhigh for the probe to receive a signal during the receiving period,after the recovery period, to enable an NQR or NMR signal emitted from asubstance within the sample to be detected.

Preferably, the reactance of the Q-factor changing means that is low isthe capacitive reactance thereof.

Preferably, the Q-factor changing means comprises a variable impedanceunit which combines with the probe coil to form a tank resonant circuitthat is capable of receiving powerful RF pulses applied to the probefrom the output of a transmitter, and permitting an RF magnetic field tobe excited in the probe coil during transmitting periods.

In this manner, the magnetic field may act on the sample and lead to theexcitation of a resonance signal in it from any traces of the targetsubstance in the sample. After the RF pulse stops, any such resonancesignal should appear in the probe and exist there together with anytransient signals (“ringing”) in the probe.

To ensure the optimum shape and duration of the powerful RF pulses, thespecific total Q-factor in the probe is preferably set by Q-factorsetting means during the transmitting period to the optimal level inorders of hundreds. For a rapid ring-down in the probe, the Q-factorduring the recovery period required for a complete damping of thetransient signals is preferably made by the Q-factor setting means to bein orders of magnitude of ones or tenths. The maximum value of the totalQ-factor that permits detection of an NQR or NMR signal is set by theQ-factor setting means during the receiving period (after the recoveryperiod) t orders of hundreds or thousand. The value of the totalQ-factor of the probe is changed by varying the impedance of the tankresonant circuit incorporating the probe coil using the variableimpedance unit.

Preferably, the Q-factor changing means is controllable to change theQ-factor of the probe in accordance with said Q-factor setting meansfurther to actively step the impedance of the probe down after saidtransmitting period to provide the requisite minimal level of theQ-factor for the probe for some period of time during the recoveryperiod.

Preferably, the impedance of the probe is stepped down to a firstminimal level of magnitude during a first period of the recovery periodimmediately following said transmitting period and then to a secondminimal level of magnitude lower than said first minimal level during asubsequent period of the recovery period, prior to said receivingperiod.

Preferably, the variable impedance unit comprises a variable stepimpedance element and switching control elements that may be activelyswitched to step the impedance elements to provide the requisiteQ-factor for the probe.

Preferably, the switching control elements are included only in thecontrol circuits of the variable step impedance element.

This permits the use of a high RF voltage in the probe that considerablyexceeds the normally permissible voltage for actively switching theswitching elements.

Alternatively, the switching elements may be placed in electrical serieswith one another and in parallel with resistive elements to enablehigher permissible voltages on the tank circuit and to achieve aplurality of possible Q-factor values during the transmitting, recoveryand receiving periods. Other elements such as a capacitor network orzener diodes may be included improving the voltage sharing duringswitching. Here, the low level drive signals need to be electricallyisolated. This may be achieved by a number of means including opticalisolation and pulse transformers.

Preferably, a balanced coil together with Q-switching elements areprovided in the probe.

Preferably, the switching control elements have a low capacitivereactance.

Preferably, a said switching control element comprises a triac orthyristor.

Preferably, the switching control elements are coupled with an inductiveelement during the period that the impedance of the probe is steppeddown to said first minimal level to protect said switching controlelements from voltage applied thereto pursuant to the resultant changein impedance, and are decoupled from said inductive element during theperiod that the impedance of the probe is stepped down to said secondminimal level to minimise the inductance of said switching controlelements and thus the reactance of the variable impedance to minimisethe impedance of the probe to change the Q-factor to said minimal levelduring the recovery period.

Preferably, the first minimal level of the Q-factor is in orders ofmagnitude of ones, and said second minimal level of the Q-factor is inorders of magnitude of tenths. In accordance with another aspect of thepresent invention, there is provided a method for detecting an NQR orNMR signal within a sample comprising:

setting the Q-factor of a probe for irradiating the sample with an RFmagnetic field pulse to an optimal level to achieve an optimum shape andduration of the RF pulse for subsequent detection of NQR or NMR signalsfrom the sample;

transmitting the RF magnetic field pulse with the probe set at saidoptimal level during the prescribed transmitting period to irradiate thesample and excite an NQR or NMR signal in the sample if a substanceproviding for NQR or NMR is present;

actively changing the Q-factor of the probe to a minimal level during aprescribed recovery period immediately following the prescribedtransmitting period; and

actively changing the Q-factor of the probe to a maximal level havinghigh orders of magnitude during the prescribed receiving periodimmediately following the prescribed recovery period, the maximal levelbeing sufficiently high for detecting the presence of any NQR or NMRsignals in a signal received from the substance during the receivingperiod;

wherein the optimal level at which the Q-factor for the probe is setduring the prescribed transmitting period is:

1(i) sufficiently low to reduce said prescribed recovery period to aperiod in which said transient signals may be dampened and to developthe leading edge of the pulse envelope of said RF pulse during saidtransmitting period; and

-   -   (ii) is also sufficiently high to reduce the bandwidth of said        RF pulse during said transmitting period and so mitigate the        power expended in amplifying the pulse envelope over the        bandwidth;

and wherein said setting and changing of the Q-factor of the probe isperformed without injecting a parasitic charge into the probe.Preferably, the method includes setting or changing the minimal level ofthe Q-factor of the probe during the recovery period to orders ofmagnitude of ones or tenths so as to be considerably lower than theQ-factor of the probe before and after the recovery period so as tocompletely dampen the transient signals and provide for a rapidring-down in the probe.

Preferably, the method includes setting or changing the maximal level ofthe Q-factor of the probe during the receiving period to orders ofmagnitude of hundreds or thousand to be considerably higher than theoptimal level of the Q-factor of the probe during the transmittingperiod and the minimal level of the Q-factor of the probe during therecovery period.

Preferably, the method includes actively stepping the impedance of theprobe down after said transmitting period to provide the requisiteminimal level of the Q-factor for the probe for some period of timeduring the recovery period.

Preferably, the method includes stepping down the impedance of the probeto a first minimal level of magnitude during a first period of therecovery period immediately following said transmitting period and thento a second minimal level of magnitude lower than said first minimallevel during a subsequent period of the recovery period, prior to saidreceiving period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) is a block diagram of a conventional NQR or NMR(magnet is not shown) apparatus for detecting a resonance signal in thespecimen.

FIG. 2 is a block diagram illustrating an NQR or NMR apparatus fordetecting a resonance signal in the specimen, according to a best modefor carrying out the present invention.

FIG. 3 illustrates graphs of waveforms seen at different positions ofthe apparatus illustrated in FIG. 2 plotted with respect to time using athree-level Q-switching arrangement, wherein:

FIG. 3 a shows a pulse produced by a transmitter unit,

FIG. 3 b shows a first control pulse produced by a control and signalprocessing unit,

FIG. 3 c shows a second control pulse produced by the control and signalprocessing unit, and

FIG. 3 d shows the Q-factor for the coil of the apparatus.

FIG. 4 shows the probe with a three-step variable impedance unit and aconventional coil, according to a first embodiment (type I) of thepresent invention.

FIG. 5 shows the probe with a three-step variable impedance unit and abalanced coil, according to a second embodiment (type I) of the presentinvention.

FIG. 6 shows the probe with a three-step variable impedance unit and aconventional coil, according to a third embodiment (type II) of thepresent invention.

FIG. 7 shows the probe with a three-step variable impedance unit and abalanced coil, according to a fourth embodiment (type II) of the presentinvention.

FIG. 8 illustrates graphs of waveforms seen at different positions ofthe apparatus illustrated in FIG. 2 plotted with respect to time using afour-level Q-switching arrangement, wherein:

FIG. 8 a shows a pulse produced by a transmitter unit,

FIG. 8 b shows a first control pulse produced by a control and signalprocessing unit,

FIG. 8 c shows a second control pulse produced by the control and signalprocessing unit,

FIG. 8 d shows a third control pulse produced by the control and signalprocessing unit, and

FIG. 8 e shows the Q-factor for the coil of the apparatus.

FIG. 9 shows the probe with a four-step variable impedance unit and aconventional coil, according to a fifth embodiment (type I) of thepresent invention.

FIG. 10 shows the probe with a four-step variable impedance unit and abalanced coil, according to a sixth embodiment (type I) of the presentinvention.

FIG. 11 shows the probe with a four-step variable impedance unit and aconventional coil, according to a seventh embodiment (type II) of thepresent invention.

FIG. 12 shows the probe with a four-step variable impedance unit and abalanced coil, according to an eighth embodiment (type II) of thepresent invention.

FIG. 13 shows the probe with a multi-step variable impedance unit and aconventional coil, according to a ninth embodiment (type I) of thepresent invention.

FIG. 14 shows the probe with a multi-step variable impedance unit and abalanced coil, according to a tenth embodiment (type I) of the presentinvention.

FIG. 15 shows the probe with a multi-step variable impedance unit and aconventional coil, according to an eleventh embodiment (type II) of thepresent invention.

FIG. 16 shows the probe with a multi-step variable impedance unit and abalanced coil, according to a twelfth embodiment (type II) of thepresent invention.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

The best mode of the present invention is directed towards an apparatusand method that causes the changing of the Q-factor of the probe todifferent Q-factors during: (i) a prescribed transmitting period whenthe apparatus is in the transmit mode, (ii) a prescribed recovery periodimmediately following the transmitting period, and (iii) a receivingperiod when the apparatus is in the receive mode immediately after therecovery period; in discrete steps.

This mode has advantages over conventional designs that are pointed outwithin the description hereinafter.

FIG. 2 is a block diagram illustrating an NQR or NMR apparatus fordetecting a resonance signal in a specimen to be irradiated with apowerful RF magnetic field pulse, according to the best mode of thepresent invention. As shown in FIG. 2, a probe 80 is connected to aconventional receiver unit 50 and a conventional transmitter unit 60 viaa duplexer and matching circuit 40. The probe 80 includes a tank circuit10 and a variable step impedance unit 20. The tank circuit 10 is tunedto a frequency of interest.

The duplexer and matching circuit 40 is a circuit which switches theprobe 80 between the transmit and receive mode as well as matches thereceiver unit 50 and transmitter unit 60 to the probe 80.

The transmitter unit 60 generates RF pulses and transfers the pulses tothe probe 80. The pulses are transmitted at a relatively high power,typically from several hundred watts to several kilowatts. These RFpulses can excite NQR or NMR signals in the specimen under investigationthat is located within the bounds of the probe 80. This signal isamplified and detected by the receiver unit 50 and is then delivered forfurther mathematical processing into a control and signal processingunit 70, the input of which is connected to the output of the receiverunit 50.

The control and signal processing unit 70 generates an RF signal, whichfrom its first output is transmitted to one of the inputs of thetransmitter unit 60 for further formation of the RF carrier for the RFpulses, and to one of the inputs of the receiver unit 50 to act as areference frequency. The control and signal processing unit 70 alsogenerates signals to another input of the transmitter unit 60 andprescribes parameters for the RF pulses and the control signals, whichare transmitted to the input of the variable step impedance unit 20 tochange or control the Q-factor of the probe 80 in discrete steps.

The control and signal-processing unit 70 usually consists of acomputer, an RF signal source for producing the RF pulses and electroniccircuits for producing the control signals, which are not specific tothe present invention and so are not described in detail here.

The apparatus shown in FIG. 2, operates in accordance with the followingmethod:

The first control signal is generated at the output of the control andsignal processing unit 70 and is input to the variable step impedanceunit 20, which sets a certain optimal Q-factor for the probe 80 duringthe transmitting period when the apparatus is in the transmit mode. TheQ-factor for the probe during the transmitting period cannot exceed andis optimally lower than the total or maximum Q for the probe during thereceive mode, which occurs immediately after the recovery period. Thetransmitter unit 60 generates RF pulses and provides the pulses to probe80 virtually simultaneously with the setting of the Q-factor for theprobe to the optimal level.

The second control signal is also generated at the output of the controland signal-processing unit 70 and is similarly sent to the variable stepimpedance unit 20. The second control signal results in setting theminimal possible Q-factor for the probe 80 during the recovery periodimmediately following the transmitting period and is lower than theoptimal Q-factor set for the probe during the transmitting period andalso than the maximal Q-factor set for the probe during the receivingperiod, immediately after the recovery period.

After the recovery period of the probe 80, a maximum possible total Qfactor is then set for the probe in order to achieving high SNR duringthe receiving period when the apparatus is in the receive modeimmediately after the recovery period. This occurs either as a result ofcontrol signals sent to the variable step impedance unit 20 beinggenerated from the output of the control and signal processing unit 70or, in some cases which will be treated in more detail below, withoutgenerating any special control signals at all. Simultaneously withsetting a high total Q factor, any NQR or NMR signal induced with theprobe is directed from the output of the probe 80 to the input of thereceiver unit 50, where it is amplified, and sent for furthermathematical processing to the control and signal processing unit 70 fordetection.

FIGS. 3(a), 3(b), 3(c) and 3(d) are diagrams illustrating, respectively,the timing of an RF pulse 90 generated by the transmitter unit 60, thefirst control signal (control pulse 1) 92 and the second control signal(control pulse 2) 93, generated by the control and signal processingunit 70, and the Q-factor of the probe 80 during these periods. FIGS.3(a), 3(b), 3(c) and 3(d) are not drawn to scale, and are included tofurther illustrate the general operation of the best mode of the presentinvention.

FIG. 3(a) illustrates RF pulse 90, which has a carrier frequency 91equal to the resonance frequency of a specimen under investigation. Thetransmitter unit 60 generates this RF pulse 90.

To set the required value of the total Q-factor of the probe 80 intransmit mode from the output of the control and signal processing unit70 to the input of the variable step impedance unit 20, the firstcontrol signal (control pulse 1) 92 is transmitted, as illustrated inFIG. 3(b). This first control signal 92 can be transmittedsimultaneously with the RF pulse 90, or earlier as indicated by theinterval 94, and is required for setting the optimal Q-factor of theprobe 80 during the transmitting period when in the transmit mode.

After the end of the RF pulse 90, the second control signal (controlpulse 2) 93 is transmitted from the output of the control and signalprocessing unit 70 to the input of the variable step impedance unit 20,as illustrated in FIG. 5(c). This second control signal pulse ensuresthat a minimal, and indeed the minimum possible, Q-factor for the probe80 is applied during the recovery period.

FIG. 3(d) illustrates the change of the Q-factor for the probe 80 duringthe transmitting period when in the transmit mode, the recovery periodand the receiving period when in the receive mode, respectively. Asillustrated in FIG. 3(d) during the action of the RF pulse 90 when inthe transmit mode 98 an intermediate level of the total Q-factor 95 ofthe probe 80 is set corresponding to the optimal level Q-factor for theprobe for generating an RF pulse of the desired character to excite NQR(or NMR) signals in the sample under investigation. After the end of RFpulse 90, during recovery period 99, the minimal Q-factor 96 for theprobe 80 is set. After the end of the recovery period during the receivemode 100, the maximum total Q factor 96 of the probe 80 is set.

Note that in FIGS. 3(b) and 3(c) a solid line shows signals whencontrol-switching elements 26 based on self-switched elements such as athyristor or a triac are used, and the interrupted line shows cases whenother actively switched elements (such as transistors or activelyswitched diodes) are used. According to the best mode for performing theinvention, the control-switching elements are in the form of elementsthat have a low capacitive reactance, such as a thyristor and a triacs.

Elements such as transistors or diodes are not used for the reason thatthey inject a parasitic charge into the probe during switching. In thepresent invention where high Q-factors are used, this parasitic chargecan cause the probe to ring anew, as previously discussed.

According to the above mode of the present invention, the use of avariable step impedance unit (as the variable step impedance unit 20)permits the operator to still use a high Q-factor coil in the probe toincrease the SNR, but in a more efficient manner that results in aconsiderable shortening of the recovery period after the effect of theRF pulses. In this manner, it is possible to control the value of theparameters of the RF pulses optimally by setting the required Q-factorfor the probe during the transmit mode.

In addition, the above mode of the present invention permits the use ofsufficiently high power RF pulses for irradiating a specimen within thebounds of the coil of the probe. The high voltage that appears then inthe tank circuit will not result in the damage to the variable stepimpedance unit.

Several practical embodiments of this mode will now be described. Theseembodiments fall into two types.

In the first type, type I, this higher voltage rating is achieved byonly including inductive elements directly within the tank circuit. Asthese are passive elements, they can tolerate very high voltages.Actively switched elements, which have limited maximum voltage capacity,are connected only to the control circuits, where the voltage can belowered to the safe value.

In the second type, type II, the higher voltage rating is achieved bysimply connecting the active switching elements in series with theparallel resistive elements allowing a selection of different Q values.

In order to illustrate the effectiveness of these embodiments, an NQRsignal emanating from a plastic explosive containing RDX was detected.This measurement used different and predetermined Q values during thetransmitting, recovery and receiving periods to optimise the systemperformance. The probe included a 260 L volume coil with a relativelyhigh Q-factor in the receive mode, in the order of hundreds to athousand, or more. Essentially, is desirable to use a probe that achievethe highest possible Q-factor, given the sensitivity required to detectan NQR or NMR signal. The duration of the recovery period was reduced bya factor of 7.5 with no measurable increase in the noise floor and thiswas achieved while still meeting the prerequisite requirements fortransmitter bandwidth and stability.

The first embodiment of the best mode of the invention is directedtowards an apparatus of the type I arrangement that provides athree-step change to the total Q-factor of a probe.

As shown in FIG. 4 the probe 80, includes a tank circuit 10 and variablestep impedance unit 20. The tank circuit and the variable step impedanceunit are configured to provide a Q-factor setting means for setting theQ-factor of the probe from minimal orders of magnitude to high orders ofmagnitude, and a Q-factor changing means having low reactance foractively changing the Q-factor of the probe without injecting aparasitic charge therein.

The Q-factor changing means actually comprises the variable impedanceunit that combines with the probe to form the tank resonant circuit. Thetank circuit is capable of receiving powerful RF pulses applied to theprobe to excite an RF magnetic field in the probe during transmittingperiods. The value of the total Q-factor of the probe is determined bythe impedance of the tank resonant circuit, whereby varying theimpedance of the variable impedance unit changes the impedance of thetank resonant circuit.

The tank circuit 10 includes a high Q coil 2, where the sample specimento be examined is placed, and a variable capacitance 1 for tuning to thepredetermined resonance frequency of the substance to be detected thatexhibits NQR properties. The variable step impedance unit 20 contains aninductive element 21 formed by a coil wound on core 23, and is connectedto the tank circuit 10. Depending on the frequency, the core 23 can bemade of iron-powder or ferrite materials.

The impedance of the variable step impedance unit 20 on the core 23 isable to be changed in discrete steps by the provision of two secondarycontrol coils 24 and 25. When these control coils 24 and 25 are switchedoff, the inductive element 21 has the highest impedance and the probe 80has the highest or maximum total Q factor that is possible, in thisembodiment 1000, which is used in the receive mode. If only one of thecoils is short-circuited (eg. control coil 24), the impedance of theinductive element 21 reduces to a predetermined optimal value, in thisembodiment 300, thus providing the required decrease in the Q-factor ofthe probe 80 during the transmitting period when in the transmit mode.When both control coils 24 and 25 are short-circuited the inductiveelement 21 has the lowest impedance and the probe 80 has the lowestQ-factor, in this embodiment 3, which is used during the recoveryperiod. To short-circuit control coils 24 and 25, control switchingelements 26 provided by triacsare used.

To ensure the highest total Q factor of the probe during the receivingperiod when in the receive mode, the impedance of the inductive element21 must not be lower than the impedance of the tank circuit 10 at theresonance frequency, but ideally be from 2 to 10 times higher. This isachieved by tuning the inductive element 21, which results in highimpedance at the frequency of interest. When using a high quality coil 2with a Q-factor of around 1000 or higher, and when the impedance of thetank circuit 10 is sufficiently high, the inductive element 21 can betuned in resonance to the required frequency to achieve a higherimpedance. For this purpose either its self-capacitance can be used, oran additional tuning capacitance 22 can be added. A skilled operator caneasily accomplish this tuning.

The number of turns in the control coils 24 and 25 can be equal ordifferent and is chosen so as to ensure the required impedance value ofthe inductive element 21 at the time of switching. The number of turnsin each of the control coils 24 and 25 can be less, equal, but ideallylarger, than the number of coils in the inductive element 21, from 2 to100 times.

The optimal level is sufficiently low to reduce the prescribed recoveryperiod to a period in which transient signals generated within the probemay be dampened and to develop the leading edge of the pulse envelope ofthe RF pulse during the transmitting period;

Importantly, the optimal level is also sufficiently high to reduce thebandwidth of the RF pulse during the transmitting period so as tomitigate the power expended in amplifying the pulse envelope over itsbandwidth.

The maximal level is as high as possible for the probe to receive asignal during the receiving period, after the recovery period, to enablean NQR or NMR signal emitted from a substance within the sample to bedetected.

The second embodiment is shown in FIG. 5 and illustrates a first variantof the probe 80 that includes a tank circuit 10 and a variable stepimpedance unit 20. The specific feature of this particular variant ofthe probe 80 is that the tank circuit 10 includes a balanced high Q coil2, which contains two similar coils 3 and 4, where the sample specimento be investigated is placed. The tank circuit 10 also includes avariable capacitance 1 for tuning to the resonance frequency of thechosen substance to be detected. The variable step impedance unit 20contains an inductive element 21, which is a coil wound on core 23 andwhich is divided into two identical coils 27 and 28, and is connected tothe tank circuit 10. Depending on the frequency, the core 23 can be madeeither of iron-powder or ferrite materials.

For changing the impedance of variable step impedance unit 20 indiscrete steps, the core 23 has four secondary control coils 29, 30, 31and 32 wound thereon. It should be noted that the control coil 29 isshort-circuited simultaneously with the coil 29, and the coil 31 isshort-circuited simultaneously with the coil 32, otherwise this variantof the probe 80 works similarly as the arrangement provided in the firstembodiment. For short-circuiting the control coils 29, 30, 31 and 32,control switching elements 26 that are conventional active switchingelements, are also used. Any suitable active switching element may beused for this purpose.

The third embodiment is directed towards an apparatus of the type IIarrangement that provides a three-step change to the Q-factor of a probeto achieve the best mode of the invention.

As shown in FIG. 6, the third embodiment illustrates a second variant ofthe probe 80, which includes a tank circuit 10 and a variable stepimpedance unit 20. The tank circuit 10 includes a high Q coil 2, wherethe sample specimen to be examined is placed and a variable capacitance1 for tuning to the resonance frequency of the substance to be detected.The variable step impedance unit 20 in the type II arrangement showncontains pulse transformers 61 to isolate the drive signals, pulseshaping networks 62 to optimise the trigger input to the activeswitching element 26, and resistive elements (R1) 63 and (R2) 64 locatedin parallel with the switching elements.

The changes in the impedance of the variable step impedance unit 20 areaccomplished via the control signals 92 and 93. The effect of closingeither switching element is to practically short-circuit the resistor inparallel. The values of the resistors should be chosen so that the sumof the resistances is sufficient to leave the Q when in the receivemode, relatively unchanged. Thus, the sum of resistances should behigher than the highest achievable impedance across any element in thetank circuit 10 and be ideally 2 to 100 times higher. Either of theresistive elements R1 63 or R2 64 can also be shorted out to reduce theQ to a predetermined ‘optimal’ value in the transmit mode. A benefit ofthis design is the possibility of achieving a higher switching voltagethan can be achieved by using a single switching element. Any suitableactive switching element may be used for this purpose.

The fourth embodiment is shown in FIG. 7 and illustrates a third variantof the probe 80, including a tank circuit 10 and a variable stepimpedance unit 20. The specific feature of this variant of the probe 80is that the tank circuit 10 includes a balanced high Q coil 2, whichcontains two similar coils 3 and 4, where the sample specimen to beinvestigated is placed. The tank circuit 10 also includes a variablecapacitance 1 for tuning to the resonance frequency of the substance tobe detected. The variable step impedance unit 20 has four activeswitching elements 71-74 in parallel with four resistive elements 81-84.A balanced coil design offers other advantages to the best mode thanthose described in previous embodiments including a further improvementin the switching voltage compared with the design of the thirdembodiment shown in FIG. 6. Typically, the switches are triggered inpairs via the control signals 100 and 101. Otherwise this variant of theprobe 80 works similarly to the third embodiment as shown in FIG. 6.Again, any suitable active switching element may be used.

The fifth to eighth embodiments are directed towards apparatus thatprovide a four-step change to the total Q-factor of the probe. These areactually preferred to the three-step change arrangements previouslydescribed.

These embodiments essentially correspond in order to the precedingembodiments and so the same reference numerals have used to indicatelike components.

Firstly with regard to FIG. 2, the four-step change arrangementessentially involves the control and signal-processing unit 70 producingthree sets of control signals, as opposed to two in the three-steparrangement. The first control signal is the same as in the three-steparrangement. However the second control signal results in setting of afirst minimal Q-factor for the probe 80 and the third control signalresults in setting of a second minimal Q-factor, which is lower than thefirst minimal Q-factor.

Essentially, the variable step impedance unit 20 is controlled to changethe Q-factor of the probe to actively step the impedance of the probedown after said transmitting period to provide the first then the secondminimal levels of the Q-factor for the probe for some period of timeduring the recovery period.

The process is better illustrated in FIG. 8 where FIGS. 8(a), 8(b),8(c), 8(d) and 8(e) are diagrams illustrating, respectively, the timingof an RF pulse 90 generated by the transmitter unit 60, the firstcontrol signal (control pulse 1) 92, the second control signal (controlpulse 2) 93 and the third control signal (control pulse 3) 93 a,generated by the control and signal processing unit 70, and the Q-factorof the probe 80 during these periods.

After the end of the RF pulse 90, the second control signal (controlpulse 2) 93 is transmitted from the output of the control and signalprocessing unit 70 to the input of the variable step impedance unit 20,as illustrated in FIG. 8(c). This second control signal pulse ensuresthat a minimal, Q-factor for the probe 80 is applied during the firstpart of the recovery period.

After this has occurred, the third control signal (control pulse 3) 93 ais transmitted from the output of the control and signal processing unit70 to the input of the variable step impedance unit 20, as illustratedin FIG. 8(d). This third control signal pulse ensures that a minimalQ-factor for the probe 80 is applied during the second part of therecovery period.

Importantly, the four-step arrangement provides for superior protectionof the control-switching elements from the high voltages that arise fromswitching from a relatively high Q-factor to a relatively low Q-factor.This allows construction of a reliable Q-switch which does notrepeatedly breakdown.

The fifth embodiment is substantially the same as the first embodiment,being directed towards an apparatus of the type I arrangement, but whichprovides for a four-step change to the total Q-factor of the probe.

As shown in FIG. 9, the apparatus is essentially the same as that of thefirst embodiment illustrated in FIG. 4, except that the impedance of thevariable step impedance unit 20 on the core 23 is able to be changed indiscrete steps by the provision of three secondary control coils 24, 25and 25 a. When these control coils 24, 25 and 25 a are switched off, theinductive element 21 has the highest impedance and the probe 80 has thehighest or maximum total Q factor that is possible, which is used in thereceive mode. If only one of the coils is short-circuited (eg. controlcoil 24), the impedance of the inductive element 21 reduces to apredetermined optimal value, thus providing the required decrease in theQ-factor of the probe 80 during the transmitting period when in thetransmit mode. When all control coils 24, 25 and 25 a areshort-circuited the inductive element 21 has the lowest impedance andthe probe 80 has the lowest Q-factor, which is used during the recoveryperiod. To short-circuit control coils 24, 25 and 25 a, controlswitching elements 26 provided by conventional active switching elementsare used. Any suitable active switching element may be used.

The number of turns in the control coils 24, 25 and 25 a can be equal ordifferent and is chosen so as to ensure the required impedance value ofthe inductive element 21 at the time of switching. The number of turnsin each of the control coils 24, 25 and 25 a can be less, equal, butideally larger, than the number of coils in the inductive element 21,from 2 to 100 times.

For changing the impedance of variable step impedance unit 20 indiscrete steps, the core 23 has six secondary control coils 29, 30, 31,31 a, 32 and 32 a wound thereon. It should be noted that the controlcoil 29 is short-circuited simultaneously with the coil 30, the coil 31is short-circuited simultaneously with the coil 32, and the coil 31 a isshort-circuited simultaneously with the coil 32 a, otherwise thisvariant of the probe 80 works similarly as the arrangement provided inthe first embodiment. For short-circuiting the control coils 29, 30, 31,31 a, 32 and 32 a, control switching elements 26 that are conventionalactive switching elements, are also used. Any suitable active switchingelement may be used for this purpose.

The switching control elements 26 have a low capacitive reactance andagain are in the form of triacs.

Importantly, the triacs 26 are coupled to inductive elements during theperiod that the impedance of the probe is stepped down to the firstminimal level to protect the triacs from voltage applied theretopursuant to the resultant change in impedance. They are then decoupledfrom the inductive elements during the period that the impedance of theprobe is stepped down to the second minimal level to minimise theinductance of the triacs and thus the reactance of the variableimpedance. This minimises the impedance of the probe to change theQ-factor to the second minimal level during the recovery period.

In the present embodiment, the first minimal level provides a Q-factorof 3 and the second minimal provides a Q-factor of 0.6.

Essentially, the first minimal level enables the voltage to be broughtdown from that generated when changing the Q-factor from the optimallevel to a level that would no longer be destructive to the triacs,using the inductive elements to protect them. The inductive elements areremoved from the triacs to reduce their reactance and thus achieve thelowest Q possible to dampen any transient signals.

The triacs have an innately long recovery period when activated again toincrease the Q-factor to the maximum. This quite advantageous as thecontrol-switching elements cannot be switched on too quickly, otherwisethey would create another signal that would be capacitively coupled withthe coil, creating ringing anew.

The sixth embodiment is substantially the same as the second embodimentand is shown in FIG. 10. For changing the impedance of variable stepimpedance unit 20 in discrete steps, the core 23 has six secondarycontrol coils 29, 30, 31, 31 a, 32 and 32 a wound thereon. It should benoted that the control coil 29 is short-circuited simultaneously withthe coil 30, the coil 31 is short-circuited simultaneously with the coil32, and the coil 31 a is short-circuited simultaneously with the coil 32a, otherwise this variant of the probe 80 works similarly as thearrangement provided in the first embodiment. For short-circuiting thecontrol coils 29, 30, 31, 31 a, 32 and 32 a, control switching elements26 that are conventional active switching elements, are also used.

The seventh embodiment is directed towards an apparatus of the type IIarrangement that provides a four-step change to the Q-factor of a probe,as shown in FIG. 11. The embodiment is substantially the same as thesecond embodiment with the variable step impedance unit 20 being in thetype II arrangement. This arrangement however, contains pulsetransformers 61 to isolate the drive signals, pulse shaping networks 62to optimise the trigger input to the active switching element 26, andresistive elements (R1) 63, (R2) 64 and (R3) 64 a located in parallelwith the switching elements.

The changes in the impedance of the variable step impedance unit 20 areaccomplished via the control signals 92, 93 and 93 a. The effect ofclosing either switching element is to practically short-circuit theresistor in parallel. The values of the resistors should be chosen sothat the sum of the resistances is sufficient to leave the Q when in thereceive mode, relatively unchanged. Thus, the sum of resistances shouldbe higher than the highest achievable impedance across any element inthe tank circuit 10 and be ideally 2 to 100 times higher. Either of theresistive elements R1 63, R2 64 or R3 64 a can also be shorted out toreduce the Q to a predetermined ‘optimal’ value in the transmit mode. Abenefit of this design is the possibility of achieving a higherswitching voltage than can be achieved by using a single switchingelement.

The eighth embodiment is shown in FIG. 12 and illustrates anothervariant of the probe 80, including a tank circuit 10 and a variable stepimpedance unit 20. The variable step impedance unit 20 has six activeswitching elements 71, 72, 73, 73 a, 74 and 74 a in parallel with sixresistive elements 81, 82, 83, 83 a, 84 and 84 a. A balanced coil designoffers other advantages to the best mode than those described inprevious embodiments including a further improvement in the switchingvoltage compared with the design of the third embodiment shown in FIG.6. Typically, the switches are triggered in pairs via the controlsignals 100, 101 and 101 a. Otherwise this variant of the probe 80 workssimilarly to the third embodiment as shown in FIG. 12. Again, anysuitable active switching element may be used.

The ninth embodiment is directed towards an apparatus of the type Iarrangement that provides multi-step changes to the Q-factor of a probeto achieve the best mode of the invention.

As shown in FIG. 13 the ninth embodiment illustrates a further variantof the probe 80, including a tank circuit 10 and a variable stepimpedance unit 20. The tank circuit 10 includes a high Q coil 2, wherethe sample specimen to be investigated is placed and a variablecapacitance 1 for tuning to the resonance frequency of the substance tobe detected. The variable step impedance unit 20 contains an inductiveelement 21, which is a coil wound on core 23, and is connected to thetank circuit 10. Depending on the frequency, the core 23 can be made ofiron-powder or ferrite materials. The difference of this variant of theprobe 80 from those described in the preceding embodiments is that itallows for changes in the Q-factor of probe 80 in multiple steps. Thisis achieved by making corresponding changes in the impedance of the core23 within the variable step impedance unit 20. Moreover, for a generalcase, there are n (3 or more) secondary control coils 33, 34 and 35.When these control coils 33, 34 and 35 are switched off, the inductiveelement 21 has the highest impedance and the probe 80 has the highesttotal Q-factor possible, which is used in the receive mode.Short-circuiting control coils 33, 34 and 35 results in decreasing thetotal Q-factor of the probe 80. For short-circuiting the control coils33, 34 and 35, control switching elements 26, which are conventionalactive switching elements, are used. Any suitable active switchingelement may be used, but in the present embodiment the element is athyristor, and more particularly, a triac.

Short-circuiting the control coils 33, 34 and 35 can be accomplishedaccording to any rule required for the optimum detection of NQR or NMRsignals, by sending corresponding control signals (control pulses) 100,101 and 102 to the inputs of control switching elements 26. This ensuresa more flexible and smooth control of the changing Q-factor for theprobe 80 during NQR or NMR signal detection. Otherwise the operation ofthis variant of the probe 80 is similar to the first embodimentdescribed with reference to FIG. 3.

The number of turns in the control coils 33, 34 and 35 can be similar ordifferent to each other and is chosen to ensure the required value ofimpedance, and changes in relation thereto, of the inductive element 21when switching. The number of turns in each of the control coils 33, 34and 35 can be less, equal but ideally higher, being from 2 to 100 timeshigher, than the number of turns in the inductive element 21.

Multi-level Q-switching may be required for performing different tasks.For instance, there maybe two Q levels at the beginning of the recoveryperiod to reduce the voltage effects upon active elements within thecircuit as in FIG. 3(e) and then this maybe followed by more two Qlevels to arrive back to the receive Q level instead of only one level.This last two Q level stage may be required to speed up the switching ofthe Q back to the best possible receive Q so that the acquisition of theweak NQR (or NMR) signal can begin sooner than would otherwise occur.

The tenth embodiment is described with reference to FIG. 14, whichillustrates another variant of the probe 80, including a tank circuit 10and a variable step impedance unit 20. The distinguishing feature ofthis variant of the probe 80 as compared to the features of the thirdembodiment as described with reference to FIG. 6, is that the tankcircuit 10 includes a balanced high Q coil 2, containing two identicalcoils 3 and 4, where the sample specimen to be investigated is placed.The tank circuit 10 also includes a variable capacitance 1 for tuning tothe resonance frequency of the substance to be detected. The variablestep impedance unit 20 contains an inductive element 21, which is a coilwound on a core 23 and which is divided into two identical coils 27 and28, and is connected to the tank circuit 10. For effecting numerouschanges in steps of the impedance of the variable step impedance unit 20on the core 23, for a general case, there are provided 2n (6 or more)secondary control coils 36, 37 and 38, 39. Short-circuiting the controlcoils 36, 37 and 38, 39 is performed by control switching elements 26,similar to those described above. Effecting the short-circuiting of thecontrol coils 36, 37 and 38, 39 can be achieved according to any rulerequired for the optimum detection by sending corresponding controlsignals (control pulses) 100, 101 and 102, 103 to the inputs of thecontrol switching elements 26. The number of turns in the control coils36, 37 and 38, 39 can be equal or different to each other, and isselected so as to ensure the required impedance value of the inductiveelement 21 at the appropriate time when they are switched. The number ofturns in each of 2n control coils 36, 37 and 38, 39 can be less, equal,but ideally larger, typically from 2 to 100 times larger than the numberof turns in the inductive element 21.

The principle of operation of the sixth embodiment of the apparatusinsofar as the probe 80 is concerned, as well as the purpose andstructural features of its other elements are similar to those previousembodiments described above.

The eleventh embodiment is directed towards an apparatus of the type IIarrangement that provides multi-step changes to the Q-factor of a probeto achieve the best mode of the invention.

As shown in FIG. 15 the eleventh embodiment illustrates another variantof the probe 80, including a tank circuit 10 and a variable stepimpedance unit 20. The tank circuit 10 includes a high Q coil 2, wherethe sample specimen to be investigated is placed and a variablecapacitance 1 for tuning to the resonance frequency of the substance tobe detected. The variable step impedance unit 20 in this case consistsof a series of active switching elements each in parallel with aresistive element and is switched by an isolated drive pulse. This drivepulse is derived from a pulse transformer and pulse-shaping network.Again, short-circuiting the resistors R1 111 to Rn 113 results indecreasing the Q-factor of probe 80. An additional benefit of thisdesign compared with previous embodiments is the possibility ofachieving a higher switching voltage than can be achieved by a singleswitching element. Any suitable active switching element may be used forthis purpose. As described in the fifth sixth embodiment with referenceto FIG. 9, the short-circuiting of the coils 36 to 39 by the activeswitching elements 26 can be accomplished according to any rule requiredfor the optimum detection of the NQR or NMR signals by sendingcorresponding control signals (control pulses) 100, 101 and 102 to theinputs of the control switching elements 26. This ensures a moreflexible and smooth control of the Q-factor of the probe 80 during NQRor NMR signal detection. Otherwise the operation of this variant ofprobe 80 is similar to the variant described in the third embodiment inrelation to FIG. 6.

The twelfth embodiment is described with reference to FIG. 16, whichillustrates a further variant of the probe 80, including a tank circuit10 and a variable step impedance unit 20. The distinguishing feature ofthis variant of the probe 80 as compared to the features of the seventhembodiment as described with reference to FIG. 10 is that the tankcircuit 10 includes a balanced high Q coil 2, containing two identicalcoils 3 and 4, where the sample specimen to be investigated is placed.The tank circuit 10 also includes a variable capacitance 1 for tuning tothe resonance frequency of the substance to be detected. The variablestep impedance unit 20 is also connected to the tank circuit 10. Foreffecting numerous (n) changes in steps of the impedance of the variablestep impedance unit 20, there exist 2n (6 or more) active switchingelements 26, in parallel with 2n resistive elements similar to thosedescribed above. Short-circuiting of the resistive elements can beperformed according to any rule required for achieving optimum detectionof the NQR or NMR signals by sending corresponding control signals(control pulses) 100, 101 and 102, 103 to the inputs of the controlswitching elements 26. A balanced coil design offers other advantagesincluding a further improvement in the switching voltage compared withthe design of the eleventh embodiment described with reference to FIG.15. Once again, any suitable active switching element may be used.

According to the eleventh and twelfth embodiments of the presentinvention, using a multi-step variable impedance unit as the variablestep impedance unit 20 provides additional advantages, namely abroadening of the scope of Q-factor switching possibilities in NQR orNMR. Thus it becomes possible to make a smoother change of the totalQ-factor of a probe during the detection process at any of its stages.It is also possible to control the required value of the total Q-factorof a probe, which usually depends on the type and size of the samplespecimen placed in the coil of the probe.

It should be appreciated, however, that the list of embodiments is notconsidered be exhaustive, and indeed the scope of the invention is notlimited thereby. For example, in all the specific embodiments of theinvention, the tank circuit 10 has been shown as a parallel resonantcircuit. In practice, other embodiments of the invention could beenvisaged and adapted to work equally as well with any tank circuit,including for example a series resonant circuit, by any one skilled inthe art.

It should also be appreciated that although the embodiments have beenspecifically described for direct application using NQR techniques,these embodiments are just as easily applied to NMR using NMRtechniques.

1. An apparatus for changing the Q-factor of a probe, having animpedance that can be varied to achieve a Q-factor of minimal orders ofmagnitude and a Q-factor of high orders of magnitude, for irradiating asample with RF energy during transmitting periods and detecting an NQRor NMR signal emitted from a substance contained within the sample froma signal received by the probe during receiving periods, the apparatuscomprising: Q-factor setting circuit for setting the Q-factor of a probefrom minimal orders of magnitude to high orders of magnitude; Q-factorchanging circuit having low reactance for actively changing the Q-factorof the probe without injecting a parasitic charge therein; and controlcircuit to control said Q-factor changing circuit so as to change theQ-factor of the probe in accordance with said Q-factor setting circuitto: (i) an optimal level during a prescribed transmitting period of anRF pulse for irradiating the sample with said RF energy; (ii) a minimallevel during a prescribed recovery period immediately following saidtransmitting period to rapidly dampen transient signals from the probe;and (iii) a maximal level during a prescribed receiving period fordetecting an NQR or NMR signal from the target substance if present,immediately following the recovery period; said optimal level being: (a)sufficiently low to reduce said prescribed recovery period to a periodin which said transient signals may be dampened and to develop theleading edge of the pulse envelope of said RF pulse during saidtransmitting period; and (b) sufficiently high to reduce the bandwidthof said RF pulse during said transmitting period and so mitigate thepower expended in amplifying the pulse envelope over the bandwidth; andsaid maximal level being sufficiently high for the probe to receive asignal during the receiving period, after the recovery period, to enablean NQR or NMR signal emitted from a substance within the sample to bedetected; wherein said control circuit is adapted to control saidQ-factor changing circuit in accordance with said Q-factor settingcircuit to actively step the impedance of the probe down in a pluralityof stages after said transmitting period for some period of time duringthe recovery period to provide the requisite minimal level of theQ-factor for the probe.
 2. An apparatus as claimed in claim 1, whereinthe reactance of the Q-factor changing circuit that is low is thecapacitive reactance thereof.
 3. An apparatus as claimed in claim 1,wherein the Q-factor changing circuit comprises a variable impedanceunit that combines with the probe to form a tank resonant circuit thatis capable of receiving powerful RF pulses applied to the probe toexcite an RF magnetic field in the probe during transmitting periods,the value of the total Q-factor of the probe being determined by theimpedance of the tank resonant circuit, whereby varying the impedance ofthe variable impedance unit changes the impedance of the tank resonantcircuit.
 4. An apparatus as claimed in claim 1, wherein the Q-factorsetting circuit is controlled to set the Q-factor of the probe duringthe recovery period to orders of magnitude of tenths to be considerablylower than the Q-factor of the probe before and after the recoveryperiod so as to completely dampen the transient signals and provide fora rapid ring-down in the probe.
 5. (canceled)
 6. An apparatus as claimedin claim 1, wherein the impedance of the probe is stepped down to afirst minimal level of magnitude during a first period of the recoveryperiod immediately following said transmitting period and then to asecond minimal level of magnitude lower than said first minimal levelduring a subsequent period of the recovery period, prior to saidreceiving period.
 7. An apparatus as claimed in claim 1, wherein saidQ-factor setting circuit comprises a variable step impedance element andswitching control elements that are able to be actively switched to stepdown the impedance of the probe to provide the requisite minimal levelof the Q-factor of the probe.
 8. An apparatus as claimed in claim 29,wherein the switching control elements are included only in the controlcircuits of the variable step impedance element.
 9. An apparatus asclaimed in claim 8, wherein said Q-factor changing circuit comprises avariable impedance unit including said variable step impedance elementthat combines with the probe to form a tank resonant circuit that iscapable of receiving powerful RF pulses applied to the probe to excitean RF magnetic field in the probe during transmitting periods, the valueof the total Q-factor of the probe being determined by the impedance ofthe tank resonant circuit, whereby varying the impedance of the variableimpedance unit changes the impedance of the tank resonant circuit, andthe switching control elements are placed in electrical series with oneanother and in parallel with resistive elements to enable higherpermissible voltages on the tank circuit and to achieve a plurality ofpossible Q-factor values during the transmitting, recovery and receivingperiods.
 10. An apparatus as claimed in claim 1, wherein said Q-factorchanging circuit comprises a variable impedance unit that combines withthe probe to form a tank resonant circuit that is capable of receivingpowerful RF pulses applied to the probe to excite an RF magnetic fieldin the probe during transmitting periods, the value of the totalQ-factor of the probe being determined by the impedance of the tankresonant circuit, whereby varying the impedance of the variableimpedance unit changes the impedance of the tank resonant circuit, andother elements comprising a capacitor network or zener diodes areincluded in the tank resonant circuit to improve the voltage sharingduring switching.
 11. An apparatus as claimed in claim 9, includingisolation circuit to electrically isolate low level drive signals forcontrolling the variable impedance unit.
 12. An apparatus as claimed inclaim 11, wherein said isolation circuit includes optical isolation andpulse transformers.
 13. An apparatus as claimed in claim 7, wherein theprobe is balanced with the switching control elements.
 14. An apparatusas claimed in claim 7, wherein said the impedance of the probe isstepped down to a first minimal level of magnitude during a first periodof the recovery period immediately following said transmitting periodand then to a second minimal level of magnitude lower than said firstminimal level during a subsequent period of the recovery period, priorto said receiving period.
 15. An apparatus as claimed in claim 7,wherein said switching control elements have a low capacitive reactance.16. An apparatus as claimed in claim 14, wherein a said switchingcontrol element comprises a triac or thyristor.
 17. An apparatus asclaimed in claim 7, wherein the impedance of the probe is stepped downto a first minimal level of magnitude during a first period of therecovery period immediately following said transmitting period and thento a second minimal level of magnitude lower than said first minimallevel during a subsequent period of the recovery period, prior to saidreceiving period, and said switching control elements are coupled withan inductive element during the period that the impedance of the probeis stepped down to said first minimal level to protect said switchingcontrol elements from voltage applied thereto pursuant to the resultantchange in impedance, and are decoupled from said inductive elementduring the period that the impedance of the probe is stepped down tosaid second minimal level to minimise the inductance of said switchingcontrol elements and thus the reactance of the variable impedance tominimise the impedance of the probe to change the Q-factor to saidminimal level during the recovery period.
 18. An apparatus as claimed inclaim 1, wherein said minimal level of the Q-factor is in orders ofmagnitude of ones or tenths.
 19. An apparatus as claimed in claim 6,wherein said first minimal level of the Q-factor is in orders ofmagnitude of ones, and said second minimal level of the Q-factor is inorders of magnitude of tenths.
 20. An apparatus as claimed in claim 1,wherein said maximal level of the Q-factor is in orders of magnitude ofhundreds or thousand. 21-31. (canceled)